1. Field of the Invention
This invention relates to a semiconductor device where a semiconductor integrated circuit is sealed with an epoxy resin; to an epoxy-resin composition with adequate properties for sealing it, in particular reduced hygroscopicity, good melt flow properties and good overall crack resistance; and to a cured product thereof. Specifically, this invention relates to an epoxy-resin composition comprising an at least bifunctional epoxy resin, in particular an epoxy resin derived from dihydroxynaphthalene, a biphenol, a novolac resin, a phenol aralkyl resin or a phenol-dicyclopentadiene resin; an at least bifunctional ester-containing compound and/or an at least bifunctional ester-containing resin as a curing agent, in particular an ester-containing resin derived from a novolac resin, a phenol aralkyl resin, a phenol-dicyclopentadiene or a naphthol aralkyl resin; and a catalyst capable of quickly reacting an ester group with an epoxy group, and a cured product thereof.
2. Description of the Related Art
An integrated circuit (IC) or a large scale integrated circuit (LSI) is practically protected by a sealer for eliminating malfunction due to dirt or dust in the outer atmosphere, heat, moisture or light.
Such a sealer has been recently shifted from a metal or ceramic to a resin, and at present sealing is mainly made with an epoxy resin.
Specifically, in the light of balance between cost and physical properties, there have been often used an epoxy-resin composition comprising a phenol resin as a curing agent. A sealer comprising an epoxy-resin composition as described above has been demanded to be improved in its mechanical properties and the following drawbacks;
(1) it absorbs moisture in the outside air and thus may be cracked due to explosive gasification of the moisture when exposed to a high temperature under soldering conditions; PA1 (2) there may partially occur homopolymerization of an epoxy monomer as a side reaction during a curing process which causes excessive hydroxy groups in a resulting phenol resin, leading to deterioration of moisture resistance and electric characteristics, as well as deterioration in mechanical characteristics due to the presence of epoxy-homopolymerization and excessive phenol-resin parts other than a desired epoxy-phenol resin network; and PA1 (3) contamination with free ions, in particular halogen ions may cause corrosion of metal parts in semiconductor or electric leakage. PA1 where B is an epoxy residue and D is a phenol residue. The hydroxyl group may increase the hydrophilicity of the resin. It may restrict reduction in an overall coefficient of moisture absorption even when the resin backbone is made hydrophobic. PA1 where R.sub.1 to R.sub.6 all of which may be the same or not are hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons or aryl or aralkyl having 6 to 10 carbons. PA1 where the ring is substituted with 2,3-epoxypropyl groups at 1, 5-, 1,6-, 1,7-, 2,6- or 2,7-positions ##STR5## PA1 where R.sub.7 all of which may be the same or not is hydrogen or methyl; ##STR6## PA1 where R.sub.8 is hydrogen, methyl or ethyl, and n is an integer of 0 to 100 whose average is 0 to 15; ##STR7## PA1 where R.sub.8 is hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons, alkoxy having 1 to 10 carbons, hydroxyl, phenyl or halogen; m is an integer of 1 to 3; and n' is an integer of 0 to 100 whose average is 0 to 50; ##STR8## PA1 where R.sub.9 is hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons, alkoxy having 1 to 10 carbons, hydroxyl, phenyl or halogen; m is an integer of 1 to 3; and n" is an integer of 0 to 50 whose average is 0 to 15. PA1 where R.sub.8 is hydrogen, methyl or ethyl; A is hydrogen or acyl where the molar ratio of hydrogen/acyl is 90/10 to 0/100; and n indicating a repeating unit number is an integer of 0 to 100 whose average is 0 to 15, provided when an average of n as a repeating unit number is zero, the formula represents bisphenol derivatives; PA1 a phenol aralkyl resin represented by general formula (8) ; ##STR10## PA1 where R.sub.9 is hydrogen, straight, branched or cyclic alkyl having 1 to 9 carbons, alkoxy having 1 to 10 carbons, phenyl, hydroxy or halogen; m is an integer of 1 to 3; n' indicating a repeating unit number is an integer of 0 to 100 whose average is 0 to 50, provided when an average of n' as a repeating unit number is zero, the formula represents bisphenol derivatives; and A is hydrogen or acyl where the molar ratio of hydrogen/acyl is 90/10 to 0/100; PA1 a phenol-dicyclopentadiene resin represented by general formula (9) ; ##STR11## PA1 where R.sub.10 is hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons, phenyl, hydroxy, halogen or alkoxy having 1 to 9 carbons; m is an integer of 1 to 3; n" indicating a repeating unit number is an integer of 0 to 50 whose average is 0 to 15, provided when an average of n" as a repeating unit number is zero, the formula represents bisphenol derivatives; and A is hydrogen or acyl where the molar ratio of hydrogen/acyl is 90/10 to 0/100; and PA1 a naphthol aralkyl resin represented by general formula (10) ; ##STR12## PA1 where n" indicating a repeating unit number is an integer of 0 to 50 whose average is 0 to 15, provided when an average of n" as a repeating unit number is zero, the formula represents bisphenol derivatives; and A is hydrogen or acyl where the molar ratio of hydrogen/acyl is 90/10 to 0/100. PA1 bisphenols such as 2,6-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), 2-(3-hydroxyphenyl)-2-(4'-hydroxyphenyl)propane, bis(4-hydroxyphenyl)methane (bisphenol F), bis(4-hydroxyphenyl)sulfone(bisphenol S), bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl)methylcyclohexane, bis(4-hydroxyphenyl)methylbenzene, 4,4'-dihydroxybiphenyl, 4,4'-dihydroxy-2,2',6,6'-tetramethylbiphenyl, 4,4'-dihydroxydiphenyl ether, 6,6'-dihydroxy-3,3,3',3'-tetramethyl-1,1-spirobiindane and 1,3,3-trimethyl-1-(4-hydroxyphenyl)-1-indan-6-ol; PA1 oligophenols such as tetraphenylolethane and naphthol-cresol resol condensate; PA1 phenol resins such as a phenol novolac represented by general formula (7), a residual moiety after removing the bisphenol from the novolac (at least triphenol; referred to as "VR"), a phenol aralkyl represented by general formula (8), a naphthol aralkyl represented by general formula (10), and a phenol-dicyclopentadiene copolymer resin (DPR resin) represented by general formula (9); PA1 aliphatic and aromatic amines such as ethylenediamine, propylenediamine, hexamethylenediamine, aniline, 4,4'-diaminophenylmethane (MDA), 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylsulfone, 2,2-bis(4,4'-diaminophenyl)propane, m-xylylenediamine, p-xylylenediamine, 1,2-diaminocyclohexane and an aniline aralkyl resin represented by general formula (1) (trade name: Anilix, Mitsui Chemical Inc.); ##STR13## PA1 where R.sub.9 is hydrogen or straight, branched or cyclic alkyl having 1 to 9 carbons; and n indicating a repeating unit number is 0 to 50 whose average is 0 to 15, provided when an average of n as a repeating unit number is zero, the formula represents a bisaniline compound; PA1 aminophenols such as m-aminophenol, p-aminophenol, 2-(4-aminophenyl)-2-(4'-hydroxyphenyl)propane and 4-aminophenyl-4-hydroxyphenyl)methane; PA1 carboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, dimer acid and 1,3-dicarboxycyclohexane; and PA1 hydroxycarboxylic acids such as salicylic acid and 4-hydroxybenzoic acid. PA1 where the ring is substituted with 2,3-epoxypropyl groups at 1, 5-, 1,6-, 1,7-, 2,6- or 2,7-positions; PA1 an epoxy resin derived from a biphenol represented by general formula (3); ##STR15## PA1 where R.sub.7 all of which may be the same or not is hydrogen or methyl; PA1 an epoxy resin derived from a novolac type of resin represented by general formula (4); ##STR16## PA1 where R.sub.8 is hydrogen, methyl or ethyl, and n is an integer of 0 to 100 whose average is 0 to 15; PA1 an epoxy resin derived from a phenol aralkyl resin represented by general formula (5) ##STR17## PA1 where R.sub.9 is hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons, alkoxy having 1 to 10 carbons, hydroxyl, phenyl or halogen; m is an integer of 1 to 3; and n' is an integer of 0 to 100 whose average is 0 to 50; and PA1 an epoxy resin derived from a phenol-dicyclopentadiene resin represented by general formula (6); ##STR18## PA1 where R.sub.9 is hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons, alkoxy having 1 to 10 carbons, hydroxyl, phenyl or halogen; m is an integer of 1 to 3; and n" is an integer of 0 to 50 whose average is 0 to 15. PA1 a novolac resin represented by general formula (7'); ##STR19## PA1 where R.sub.8 is hydrogen, methyl or ethyl; and n indicating a repeating unit number is an integer of 0 to 100 whose average is 0 to 15, such as a phenol novolac resin and a cresol novolac resin, provided when an average of n as a repeating unit number is zero, the formula represents a bisphenol compound; PA1 a residual moiety after removing the bisphenol from the novolac (at least triphenol; referred to as "VR"); PA1 a phenol aralkyl resin represented by general formula (8') ##STR20## PA1 where R.sub.9 is hydrogen, straight, branched or cyclic alkyl having 1 to 9 carbons, phenyl, hydroxy, halogen or alkoxy having 1 to 10 carbons; m is an integer of 1 to 3; n' indicating a repeating unit number is an integer of 0 to 100 whose average is 0 to 50, provided when an average of n' as a repeating unit number is zero, the formula represents a bisphenol compound; PA1 where R.sub.10 is hydrogen, straight, branched or cyclic alkyl having 1 to 10 carbons, phenyl, hydroxy, halogen or alkoxy having 1 to 9 carbons; m is an integer of 1 to 3; n" indicating a repeating unit number is an integer of 0 to 50 whose average is 0 to 15, provided when an average of n" as a repeating unit number is zero, the formula represents a bisphenol compound; and PA1 a naphthol aralkyl resin represented by general formula (10'); ##STR22## PA1 where n" indicating a repeating unit number is an integer of 0 to 50 whose average is 0 to 15, provided when an average of n" as a repeating unit number is zero, the formula represents a bisphenol compound;
Ionic impurities as described in (3) may be controlled by refining the epoxy resin; (1) and (2) may be controlled by reforming the resin and minimizing side reactions, respectively, to allow the desired physical properties for an epoxy-resin composition to be adequately attained.
Common sealers recently used for a semiconductor include a relatively inexpensive combination of o-cresol novolac type of epoxy resin (trade name: EOCN-102S, Nihon Kayaku, etc.) and a phenol novolac resin (trade name: BRG#558, Showa Kobunshi, etc.) as a universal grade sealer; and a combination of biphenol or a tetramethylbiphenol type of epoxy resin (trade name: YX4000, Yuka Shell Epoxy, etc.) and a phenol aralkyl resin (trade name: MilexXLC-4L, Mitsui Chemical Inc., etc.) as a high grade sealer.
The former combination is characterized in improved heat resistance by the use of an epoxy resin and a curing agent having a higher functional group density and a lower cost, but has a drawback to be improved that its higher functional group density makes it highly hygroscopic and its higher melt viscosity reduces the limit of its filler filling rate. The latter combination is characterized in improved workability because of combining a crystalline epoxy resin having a melting point of near 100.degree. C. and having a reduced melting viscosity and a lower hygroscopic and flexible phenol aralkyl resin, and improved crack resistance because of its lower hygroscopicity, but has been demanded not only to reduce a cost, but also to solve the problem that its heat resistance is deteriorated due to reduction in its crosslink density caused by using a mainly bifunctional epoxy resin.
In terms of the problem (1) of moisture absorption of a resin, as long as curing is a reaction between an epoxy group with a hydroxyl group, the curing reaction inevitably produces a hydroxyl group as illustrated by the equation below; ##STR2##
To solve the problem, JP-A 62-53327 by Nishikubo et al. has suggested a certain reaction of an epoxy group with an ester group.
In the specification, quaternary onium salts and crown ether complexes have been listed as a preferable catalyst. In addition, they have shown yields when using the specific catalysts in a unit reaction, in their paper (Nishikubo et al., Yuki Gosei Kagaku, vol.49, 218-233 (1991), "An addition reaction of an epoxy compound with an ester and its applications in polymer synthesis". According to the paper, tetrabutylammonium chloride gives the highest yield of 91% but the yield are generally low. When remaining in a resin as a sealer for a semiconductor integrated circuit, a quaternary onium salt or crown ether may, of course, cause not only undesirable effects such as electric short-circuit, but also erosion of metal parts in contact with the sealer, leading to a major defect.
In an addition reaction of an epoxy resin with a phenol resin, catalysts used include phosphines such as trialkylphosphines and triarylphosphines; imidazoles; and tertiary amines; particularly imidazoles and phosphines are often employed for sealing a semiconductor.
It is, however, known that an imidazole compound is highly reactive, but tends to cause the epoxy-homopolymerization, leading to the problem (2). On the other hand, a phosphine does not cause such a problem, but cures too slowly.
When applying an imidazole compound as a catalyst to an epoxy/ester curing reaction, a desired addition reaction of an ester group to an epoxy group may occur in a reaction yield of about 50% and otherwise there may occur side reactions such as epoxy homopolymerization, according to the paper by Nishikubo et al. Thus, the catalyst does not give an acceptable cured product.
According to our replication, it has been found that when using an imidazole or phosphine compound as a curing catalyst, an ester-containing resin esterified by an acyl group of this invention does not essentially react with an epoxy resin to give a cured product.
Specifically, the resin composition fails to be gelled for more than 10 min at 150 to 200.degree. C. which is a temperature range usually used for curing, and thus do not provide a cured product, but in reality the resin composition will be flown out (see, Comparative Examples later).
JP-A 9-235451 has been suggested a process where 10 to 90% of a phenol resin is esterified, which is then used for ester-curing of an epoxy resin to give a sealer for a semiconductor integrated circuit.
The process is based on a concept that some of phenolic hydroxy groups in the starting phenol resin are left unreacted and thus the reactive phenolic moieties first form crosslinks in an initial curing step and the product is then after-cured by reacting its epoxy groups with ester groups, to give an ester resin.
The specification has disclosed phosphines, imidazoles and diazabicyclo compounds as a curing catalyst. However, phosphines does not exhibit adequate performance as a curing catalyst for an epoxy and an ester group as shown herein (see, Comparative Examples later); and imidazoles and diazabicyclo compounds cause epoxy homopolymerization to a significant degree, makes it difficult to adjust a molar ratio of epoxy groups to functional groups in the curing agent, and adversely affects product's physical properties as indicated in the paper by Nishikubo et al. In addition, the specification has indicated that a compound esterified with an aliphatic acyl group is not preferable for reaction with an epoxy group.